1. Field of the Invention
The present invention generally relates to an ultrasonic diagnosis apparatus and an ultrasonic diagnosis method, and more particularly, to an ultrasonic diagnosis apparatus and an ultrasonic diagnosis method which are used to capture and display ultrasonic images of an object with the use of ultrasonic waves, and are used in diagnosing tissues.
2. Description of the Related Art
Conventionally, a sound velocity value in a portion (a diagnosed site) in an object (such a sound velocity value will be hereinafter referred to as a local sound velocity value) is measured with the use of ultrasonic waves. For example, the following methods have been suggested. According to one of the methods, two oscillators for transmission and reception are positioned to face each other, and a sound velocity value in an object is calculated from the distance between the oscillators and the period for propagation of ultrasonic waves between the oscillators. According to another one of the methods, two sets of oscillators arranged at predetermined intervals are used for transmission and reception, and a propagation velocity is calculated from the period for ultrasonic propagation between the oscillators, transmission and reception angles, and the distances between the oscillators in the respective pairs.
Japanese Patent Application Laid-Open No. 5-95946 discloses the following method of measuring values of local sound velocities. According to Japanese Patent Application Laid-Open No. 5-95946, ultrasonic waves are transmitted while the output angle from a transmission oscillator toward an object is being varied, and ultrasonic waves are received while the incident angle is being varied by a reception oscillator. All the periods of time elapsed from transmission to reception are stored into a memory. A hypothetical distribution of sound velocities is set, and, based on the distribution of sound velocities, the elapsed time is calculated for each of the output angles and incident angles. The hypothetical distribution of sound velocities is then corrected so that the differences between the calculated values of period of elapsed time and actual measurement values are minimized. The sound velocity value in the object is then determined from the resultant distribution of sound velocities.
A sound velocity value V in an object OBJ1 made of a medium having a constant sound velocity value can be calculated in the following manner. As shown in FIG. 7A, where L represents the distance from a reflection point (region) X1ROI in the object OBJ1 to an ultrasonic probe 300A, the period of time T elapsed from reflection of ultrasonic waves at the reflection point X1ROI to reception of the ultrasonic waves by an element 302A0 located immediately below the reflection point X1ROI is expressed as T=L/V. Where the period of time elapsed before reception by an element 302A1 located at a distance X in the X-direction (the array direction of elements 302A) from the element 302A0 is expressed as T+ΔT, the delay time ΔT between the elements 302A0 and 302A1 is expressed by the following mathematical formula (1):ΔT=ΔL/V (where ΔL=√(L2+X2)−L)  (1)
Therefore, after the ultrasonic waves are reflected at the reflection point X1ROI the time T after the transmission of the ultrasonic waves, the periods of time 2T and 2T+ΔT elapsed before the ultrasonic waves are received by the element 302A0 and another element are measured. In this manner, the distance L to the reflection point X1ROI and the velocity V can be uniquely determined.
Where the ultrasonic waves reflected from the reflection point X1ROI can be clearly recognized, the distance L and the velocity V can be determined from the elapsed time measured at the element 302A0 and another element. However, ultrasonic detection signals which are output from the respective elements 302A are normally resulted from interferences by signals from numerous reflection points, and it is difficult to distinguish only the signals supplied from a specific reflection point. Therefore, in practice, the distance L to the reflection point X1ROI, the delay time ΔT, and the sound velocity value V are uniquely determined from the spatial frequency, sharpness, and contrast in a re-formed image in a region of interest in the vicinity of the reflection point X1ROI.
As described above, where the sound velocity in an object is uniform, the value of the sound velocity can be calculated. When the internal sound velocity is not uniform as in an object OBJ2 shown in FIG. 7B, however, it is difficult to calculate the distance L to a reflection point (region) X2ROI and the sound velocity values V and V′ by the above described method.
To counter this problem, the applicant suggested a method of determining a local sound velocity where the sound velocity in an object is not uniform (Japanese Patent Application Laid-Open No. 2010-99452). According to this method, a hypothetical sound velocity is set in a region of interest in an object, and an optimum sound velocity value at a lattice point set in a shallower region than the region of interest is set. Based on the hypothetical sound velocity and the optimum sound velocity value, an image of the region of interest is generated from reception signals of the respective elements obtained when ultrasonic waves are transmitted to the region of interest, and the image of the region of interest is then analyzed. Alternatively, an optimum sound velocity value or a reception wave at a representative lattice point in the region of interest is calculated. The local sound velocity value in the region of interest is then determined by comparing the hypothetical sound velocity in the region of interest with the optimum sound velocity value or the reception wave calculated based on the optimum sound velocity value set in the shallower region than the region of interest. As described above, the method suggested previously by the present applicant is a method of approximating the reception wave at each lattice point by the ambient sound velocity (an optimum sound velocity), and this method enables measurements of local sound velocities even where the sound velocity in the object is not uniform.
Where the unsteadiness of the sound velocity in the object is higher than predicted, however, the above described method of approximating the reception wave at each lattice point by the ambient sound velocity cannot cope with the unsteadiness of the sound velocity.